Fusion draw process is used to make a sheet of material from molten material, such as molten glass. The general fusion draw process is described in U.S. Pat. Nos. 3,338,696 and 3,682,609, both issued to Dockerty. Typically, the fusion draw process involves delivering molten material into a trough and overflowing the molten material down the sides of the trough in a controlled manner. The separate streams of material flowing down the sides of the trough merge at the root of the trough into a single stream of material, which is drawn into a continuous sheet of material. The continuous sheet of material is separated into discrete pieces at the bottom of the fusion draw machine. A key advantage of this process is that the surfaces of the sheet of material do not come in contact with the sides of the trough or other forming equipment and therefore are pristine. Another benefit of the process is that the sheet of material is very flat and has a uniform thickness.
Large sheets of glass produced by fusion draw process are a key component in making large flat panel displays. Alternatively, they can be diced to make other devices such as active electronic devices, photovoltaic devices, and biological arrays. However, as the demand for even larger sheets of material increases, so does the difficulty in producing and handling of these sheets. For example, sheet scoring and separation processes at the bottom of the fusion draw machine (FDM) contribute significantly to the sheet motion in the forming zone of the FDM. Sheet motion in the forming zone can negatively impact the level of stress and stress variation within the sheet, possibly leading to distortion in the final product. The larger the sheet being handled, the more significant the effect of sheet motion can be on the stress level and variation with the sheet.
Corning Incorporated, the assignee of the present invention, has developed various techniques for minimizing sheet motion at the bottom of the draw. One such technique involves scoring the glass sheet by laser, thereby avoiding physical contact with the glass sheet that can result in sheet motion, as described in U.S. patent application Ser. No. 12/008,949. Another technique involves use of a conformable nosing device to engage a glass sheet while the glass sheet is being scored, thereby reducing motion of the glass sheet during scoring, as described in U.S. patent application Publication No, US2008/0276646. Another technique involves separation of the glass sheet without bending the glass sheet, as described in U.S. Patent Application Publication No. US2007/0039990. These techniques require real-time information about the position of selected areas of the glass sheet relative to a reference plane. Such information at different elevations of the FDM is also useful in fine-tuning and optimizing the draw process.